In previous practice of the art phosphate rock or bone phosphate is sometimes treated with sufficient sulfuric acid to make phosphates "available" as plant or animal nutrient without separating the calcium sulfate formed. Alternatively, as in the wet acid process, acidulation with sulfuric acid can be complete to yield phosphoric acid, which is separated from the calcium sulfate residue, to use as a fertilizer and for other common uses of phosphoric acid, or is combined with other plant food materials, such as nitrogen and potassium compound to make mixed fertilizers.
A high quality phosphoric acid is produced by the reaction of calcium phosphate raw materials with silica and carbon in an electric arc furnace to produce elemental phosphorous, which is then burned, and the phosphorous oxides formed absorbed with water to produce high-quality phosphoric acid.
There is also mentioned in a prior patent the reaction of phosphate rock and calcium fluoride with sulfur trioxide at elevated temperatures to produce phosphorous oxyfluoride, which hydrolyzes to phosphoric acid and hydrofluoric acid. (U.S. Pat. No. 3,402,019.)
All these processes of making phosphate "available" as a nutrient, or phosphoric acid, have economic drawbacks.
The acidulation of insoluble phosphates with sulfuric acid is primarily of use for making nutrients available to plant and animals. If the acidulation is only partial and the phosphoric content is not removed, the mass of the residue makes the cost of transportation and handling relatively high, so that for the most part, these types of materials, of which super-phosphate is typical, are used only in areas close to the point of manufacture. Another disadvantage to such process is that the reaction is slow and requires long residence time.
In the so-called wet process, where phosphoric acid is made by contacting insoluble phosphates with dilute sulfuric acid, very expensive and large plants are required because of the corrosiveness of the dilute acids used, the intricate stages and recycles necessary for proper crystal growth to facilitate the separation of the phosphoric acid from the calcium sulfate residue, and the necessity of evaporating large amounts of excess water from the phosphoric acid product to handle and ship economically.
Phosphoric acid made via the elemental phosphorous route has an even higher cost of manufacture due to the electrical energy consumed and the very high temperature furnace equipment needed, so that the product is primarily used only as a food ingredient.
The method of making phosphoric acid using sulfur trioxide and calcium fluoride has found little, if any, commercial application, probably due to the added cost of the calcium fluoride, the increased bulk of calcium sulfate residue, and the large amount of fluorine by-products.